Support transient input methods in Isearch mode (bug#44266)

Support transient input methods in Isearch mode (bug#44266)

* doc/emacs/mule.texi (Select Input Method): Rename
transient-input-method to activate-transient-input-method.

* doc/emacs/search.texi (Special Isearch):
Document isearch-transient-input-method.

* lisp/international/isearch-x.el (isearch-transient-input-method):
New function.
(isearch-process-search-multibyte-characters):
Call 'deactivate-transient-input-method' after 'read-string'.

* lisp/international/mule-cmds.el (mule-menu-keymap): Remove
duplicate menu item 'describe-input-method'.  Add new menu item
'activate-transient-input-method'.
(default-transient-input-method): Rename from transient-input-method.
(current-transient-input-method)
(previous-transient-input-method): New buffer-local variables.
(deactivate-input-method): Don't add
current-transient-input-method to input-method-history.
(toggle-input-method): Call deactivate-transient-input-method
when current-transient-input-method is non-nil.
(activate-transient-input-method): Rename from transient-input-method.
(deactivate-transient-input-method): New function with body from
renamed function transient-input-method.

* lisp/isearch.el (isearch-menu-bar-map): Add new menu item
'isearch-transient-input-method'.
(isearch-mode-map): Bind 'C-x \' to isearch-transient-input-method.
(isearch-forward): Add isearch-transient-input-method to docstring.
(isearch-message-prefix): Use shorter string for narrowed buffer.